Vanadocene-Mediated Ionization of Water in the Aqua Species[H2O‚B(C6F5)3]: Structural Characterization of the Hydride and

Hydroxide Complexes [Cp2V(µ-H)B(C6F5)3] and[Cp2V(µ-OH)B(C6F5)3]

Robert Choukroun,* Christian Lorber,† Laure Vendier, and Christine Lepetit

Laboratoire de Chimie de Coordination, CNRS UPR 8241, 205 route de Narbonne,31077 Toulouse Cedex 04, France

ReceiVed NoVember 10, 2005

Summary: The reaction of Cp2V with B(C6F5)3 in the presenceof water leads to the hydride complex [Cp2V(µ-H)B(C6F5)3] (1)and the hydroxide complex [Cp2V(µ-OH)B(C6F5)3] (2), both ofwhich haVe been characterized by X-ray structure determina-tions. Both complexes are deriVed from the ionization of waterin the presence of B(C6F5)3.

Various aspects of the chemistry and applications of B(C6F5)3

have been recently reviewed.1,2 Though B(C6F5)3 is well-knownas an activator for homogeneous group 4 transition-metal-catalyzed olefin polymerization,3 it displays versatile chemistryand unique reactivity. It is highly reactive with water, and itsaqua complex H2O‚B(C6F5)3 is known to behave as a strongBrønsted acid.4 Indeed, adventitious water has given access tothe hydroxyborate complexes [M][O(H)B(C6F5)3] (M ) Ti, Zr).5

The identification of the hydroxyborate anions [HOB(C6F5)3]-,[(F5C6)3B(µ-OH)B(C6F5)3]-, and [(F5C6)3B(Η3O2)B(C6F5)3]-,which are derived from the reaction of B(C6F5)3 with water,suggests the presence of an acidic-proton-containing counter-cation.6 Several reactions reported recently in the literatureindirectly support the presence of such a cation; treatment of[MCp2] (M ) Cr, Fe, Co) with H2O‚B(C6F5)3 resulted in metaloxidation with H2 elimination.7 The hydride species [M(µ-H)B-(C6F5)3] (M ) Ti, Zr, Nb) has been suggested to be the keyintermediate. In some cases the intermediate has been observedby NMR spectroscopy,8 but it has never been isolated andstudied. Here we report the first isolation of this hydride speciesin a vanadocene system.

Addition of Cp2V in toluene to a solution of hydrated borane9

gives a blue-purple solution and, after 1 day, yellow-orange andblue crystals (Scheme 1). After hand separation, both compounds

were unambiguously characterized by an X-ray structureanalysis. Figures 1 and 2 give ORTEP views of the molecularstructures of1 and2, respectively.

* To whom correspondence should be addressed. E-mail:choukroun@lcc-toulouse.fr.

† E-mail: lorber@lcc-toulouse.fr.(1) Piers, W. E.AdV. Organomet. Chem. 2005, 52, 1-76.(2) Erker, G.Dalton Trans. 2005, 1883-1890.(3) For a recent review on this subject see: Chen, E. Y-X.; Marks, T.

J. Chem. ReV. 2000, 100, 1391-1434.(4) (a) Beringhelli, T.; Maggioni, D.; D’Alfonso, G.Organometallics

2001, 20, 4927-4938. (b) Bergquist, C.; Bridgewater, B. M.; Harlan, C.J.; Norton, J. R.; Friesner, R. A.; Parkin, G.J. Am. Chem. Soc.2000, 122,10581-10590.

(5) (a) Spanneberg, A.; Burlakov, V. V.; Arndt, P.; Baumann, W.; Shur,V. B.; Rosenthal, U.Z. Kristallogr. 2002, 217, 546-548. (b) Choukroun,R.; Lorber, C.; Vendier, L.Eur. J. Inorg. Chem. 2004, 317-321.

(6) (a) In presence of the “metal base” [Ir(Cp)(C8H12)], a salt containingthe hydride ligand on iridium, [Ir(Cp)(C8H12)H][(F5C6)3B(µ-OH)B(C6F5)3],was isolated.4b (b) Danopoulos, A. A.; Galsworthy, J. R.; Green, M. L. H.;Cafferkey, S.; Doerrer, L. H.; Hursthouse, M. B.Chem. Commun.1998,2529-2530. (c) Di Saverio, A.; Focante, F.; Camurati, I.; Resconi, L.;Beringhelli, T.; d’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.;Sironi, A. Inorg. Chem. 2005, 44, 5030-5041.

(7) Doerrer, L. H.; Green, M. L. H.J. Chem. Soc., Dalton Trans.1999,4325-4329.

(8) (a) Harlan, C. J.; Hascall, T.; Fujita, E.; Norton, J. R.J. Am. Chem.Soc.1999, 121, 7274-7275. (b) Plecnik, C. E.; Liu, F.-C.; Liu, J.; Meyers,E. A.; Shore, S. G.Organometallics2001, 20, 3599-3606. (c) Yang, X.;Stern, C. L.; Marks, T. J.Angew. Chem., Int. Ed. Engl.1992, 31, 1375-1377. (d) Liu, F. C.; Liu, J.: Meyers, E. A.; Shore, S. G.J. Am. Chem.Soc.2000, 122, 6106-6107. (e) Liu, S.; Liu, F.-C.; Renkes, G.; Shore, S.G. Organometallics2001, 20, 5717-5723.

(9) Prepared by the addition 2 equiv of B(C6F5)3 with 1 equiv of H2O orfrom isolated H2O‚B(C6F5)3) (1 equiv) with B(C6F5)3 (1 equiv).

Scheme 1

Figure 1. Ortep drawing of the molecular structure of1 showing50% probability ellipsoids and partial atom-labeling schemes.Hydrogen atoms are omitted for clarity. Selected parameters (bondlengths in Å and angles in deg): V1-H100 ) 1.87(2), B1-H1)1.34(2); V1-H100-B1 ) 152.8, Cp1-V1-Cp2 ) 142.8 (Cp1and Cp2 are the centroids of the C1-C5 and C6-C10 Cp ligands,respectively).

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The yellow-orange crystals were determined to be the hydridecomplex [Cp2V(µ-H)B(C6F5)3] (1). The data were of sufficientlyhigh quality that we were able to locate a hydrogen atombetween the vanadocene moiety and the borane Lewis acid. TheV-H distance in1 (1.87(2) Å) is comparable to those incomplexes having an hydrogen atom bridging a vanadium centerand a second metal (V, Zn) or a boron atom of aη1- or η2-borohydride ligand (V-H ) 1.69(4)-1.94(3) Å).10 The B-Hdistance in1 (1.34(2) Å) is longer than that found in theaforementionedη1- andη2-BH4 complexes (1.12 (av) and 1.10Å, respectively). This B-H bond is also longer than in the anion[HB(C6F5)3]- (for example, for [Cp2V(CO)2][HB(C6F5)3] B-H) 1.14(2) Å11 and for [Cp*2Zr][HB(C6F5)3] B-H ) 1.06(6)Å8c). It is worth noting that the sums of the angles C-B-Caround the boron atom are nearly the same in1 and in the[HB(C6F5)3]- anion.

A comparative topological analysis was performed using theelectron localized function (ELF)12 in order to get more insightinto the V-H-B interaction in1. The ELF picture of1 wascompared to those of the free B-H bond in the [HB(C6F5)3]-

anion, free V-H in Cp2VH, and V-H-V in Cp4V2H2. As nocrystallographic structure was available for the last two com-plexes, their structures were calculated at the B3PW91/6-31G*level (see the Supporting Information). According to the ELFanalysis of1, the H atom interacts with both V and B, as it isinvolved in a three-center-two-electron bond (defined in theELF by the trisynaptic basinV(V,H,B)). The atomic contributionof V to the population of the trisynaptic basinV(V,H,B) is closeto but slightly lower for1 than for Cp4V2H2. Similarly, theatomic contribution of the B atom to the population ofV(V,H,B)is slightly lower for1 than for the [HB(C6F5)3]- anion. Thesefindings are in agreement with the structural parameters, which

suggest a bridging hydride in1 with weakened V-H or B-Hbonds. In a sense,1 can be regarded as a vanadium hydridestabilized by a borane before its abstraction by the borane,leading to a salt containing the [HB(C6F5)3]- anion.8d,13

The blue crystals were structurally characterized as being theexpected [hydroxytris(pentafluorophenyl)borato]vanadium(III)complex [Cp2V(µ-OH)B(C6F5)3] (2), in which the hydrogenatom of the hydroxide group was also located by the X-raystructure determination. The V-O (2.0875(14) Å) and O-B(1.518(3) Å) distances are those expected for a bridginghydroxide of a zwitterionic complex. In contrast to the case forthe analogous titanium complexes [Cp2Ti(µ-OH)B(C6F5)3] and[rac-(ebthi)Ti(µ-OH)B(C6F5)3], in which a C-F‚‚‚Ti interactionbetween the Ti center and an ortho fluorine atom of the boranemoiety occurs in the solid-state structure,5 no such interactionis observed in the vanadium complex2.

According to the stoichiometry of the reaction (Scheme 1)compounds1 and2 should be obtained in equimolar amounts.However, only a low yield of1 is obtained. Varying theexperimental (solvent, concentration of reactants) or workupconditions did not improve the yield of isolated1 significantly,and as a consequence this precluded its full characterizationand its reactivity study.13 Furthermore, meticulously driedaprotic solvent added to a mixture of1 and 2 immediatelyemitted fumes and led to partial decomposition of1 (leaving2unchanged, as monitored by IR). The very high sensitivity of1toward adventitious water may explain the difficulty weencountered in its isolation and may also explain why suchhydride species have remained elusive in other metal systemsthat have been studied. The magnetic moments measured for1and2 are in agreement with those of vanadium(III) species (µeff

) 2.85 and 2.95µB, respectively).13 TheνOH infrared frequencyof 2 in KBr is observed at 3614 cm-1. The deuterated analogue[Cp2V(µ-OD)B(C6F5)3]14 has aνOD value of 2666 cm-1. For 1(which was always contaminated by a small amount of2) themedium-broad B-H stretching bands centered at 1904 cm-1

are too complex to be assigned but are compatible with amonodentate borohydride species;15 the B-D stretching bandof the deuterated analogue is not observed, due to its superposi-tion with other bands of the complex.

The concomitant formation of a hydride species1 and ahydroxide species2, starting from [H2O][B(C6F5)3], implies thetwo-electron reduction of H2O by Cp2V. We suggest the reactionto proceed via the overall pathway depicted in Scheme 2, whichinvolves the intermediacy of a V(IV) species ([Cp2VH]+[HOB-(C6F5)3]-) that further evolves through disproportionation withCp2V into the vanadium(III) complexes1 and2.16

(10) (a) Jonas, K.; Wiskamp, V.; Tsay, Y.-H.; Kru¨ger, C.J. Am. Chem.Soc. 1983, 105, 5480-5481. (b) Bansemer, R. L.; Huffman, J. C.; Caulton,K. J.J. Am. Chem. Soc.1983, 105, 6163-6164. (c) Berno, P.; Gambarotta,S. Angew. Chem., Int. Ed. Engl.1995, 34, 822-824. (d) Gerlach, C. P.;Arnold, J.J. Chem. Soc., Dalton Trans.1997, 4795-4805. (e) Jensen, J.A.; Girolami, G. S.Inorg. Chem.1989, 28, 2107-2113.

(11) Choukroun, R.; Lorber, C.; Donnadieu, B.Organometallics2004,23, 1434-1437.

(12) Silvi, B.; Savin, A.Nature1994, 371, 683-686.

(13) The spectroscopic and magnetic studies were conducted at roomtemperature on a pure crystalline sample obtained after separation by handof a mixture of large crystals of1 and 2. Spectroscopic studies of1 inTHF-d8 showed the formation of the vanadium(III) EPR-silent salt [Cp2V]+-[HB(C6F5)3]-: 1H NMR (400 MHz) 145 (br, Cp), 4.28 ppm (br q, BH,JBH) 80 Hz);11B NMR -24.5 ppm (d,JBH ) 76 Hz);19F NMR -46.2,-83.9,-85.3 ppm. Complex2 is EPR-silent. Spectroscopic studies of1 in THF-d8: 1H NMR (400 MHz) 134 ppm (br, Cp) (HO unobserved);11B NMR-3.3 ppm;19F NMR unresolved. (note:19F NMR (188.3 MHz), referenceCF3CO2H; 11B NMR (160.5 MHz), reference BF3‚Et2O).

(14) [Cp2V(µ-OD)B(C6F5)3] was obtained from the above reactionscheme using D2O in place of H2O.

(15) (a) Marks, T. J.; Kolb, J. R.Chem. ReV. 1977, 77, 263-293. (b)Marks, T. J.; Kennelly, W. J. J. Am. Chem. Soc.1975, 97, 1439-1443.

Figure 2. Ortep drawing of the molecular structure of2 showing50% probability ellipsoids and partial atom-labeling schemes.Hydrogen atoms are omitted for clarity. Selected parameters (bondlengths in Å and angles in deg): V1-O1 ) 2.0875(14), B1-O1) 1.518(3), O1-Ho1) 0.75(2); V1-O1-B1 ) 142.57(13), Cp1-V1-Cp2 ) 141.7 (Cp1 and Cp2 are the centroids of the C1-C5and C6-C10 Cp ligands, respectively).

Scheme 2

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We have already reported the formation of a hydride species(at that time wrongly formulated as the salt [Cp2V][HB(C6F5)3])from the reaction of [Cp2VCO] with B(C6F5)3.11 We can nowsee that redox and disproportionation reactions afford fourdifferent complexes:1, the structurally established, unprec-edented, zwitterionic ring-borylated vanadium(III) complexesCpCpBV and CpCpBV(CO)2 (CpB ) η5-C5H4B(C6F5)3), and theionic vanadium(III) species [Cp2V(CO)2][HB(C6F5)3].

In conclusion, the existence of complex1 is of considerableimportance and contributes to a better understanding of ionpairs.17 It shows for the first time that such a hydride intermedi-ate postulated for early-transition-metal complexes exists, and

we have demonstrated that its formation implies a two-electronreduction of water (mediated, in our work, by Cp2V). Due tothe powerful water scavenging ability of B(C6F5)3, one maynot exclude the possibility that such hydride and hydroxidespecies can be formed during catalytic olefin polymerizationprocesses involving metallocenes and B(C6F5)3 cocatalyst in thepresence of traces of water.1 We are currently working to answerthis point.

Acknowledgment. We are grateful to the CNRS for financialsupport.

Supporting Information Available: CIF files giving crystaldata for complexes1 and2 and tables giving computational details.This material is available free of charge via the Internet athttp://pubs.acs.org.

OM050965X

(16) Disproportionation and redox reactions are often occurring invanadium chemistry; see for example: (a) Choukroun, R.; Lorber, C.Eur.J. Inorg. Chem.2005, 4683-4692. (b) Choukroun, R.; Moumboko, P.;Chevalier, S.; Etienne, M.; Donnadieu, B.Angew. Chem., Int. Ed.1998,37, 3169-3172. (c) Berno, P.; Gambarotta, S.; Richeson, D.ComprehensiVeOrganometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,Eds.; Pergamon: New York,1995; Vol. 4, pp 1-55. (17) Luo, L.; Marks, T. J.Top. Catal.1999, 7, 97-106.

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